Mass spectra of elements
For an element, the mass spectrum shows the mass of all the isotopes and the
relative amount of each. For example, here is the mass spectrum of neon.
Mass spectroscopy provides an accurate way of measuring the mass of atoms and
molecules.
20
90.9 %
21
0.26 %
22
8.8 %
Ne
Relative
abundance
Ne
Ne
strong electromagnet
Ar = [ (90.9 x 20) + (0.26 x 21) + (8.8 x 22) ] = 20.17
(90.9 + 0.26 + 8.8)
19
sample
20
21
22
23
m/z
detector
electric field to
accelerate and focus
lithium
6Li
7Li
gallium
69Ga
(1.00)
iron
54Fe
(5.8%)
(7.4%)
(92.6%)
electron gun
1)
2)
71Ga
(0.66)
Ionisation
56Fe
(91.6%)
57Fe
58Fe
(2.2%)
(0.3%)
Acceleration
Mass spectra of molecules
3)
For elements that are made of
molecules, peaks will be seen for the
molecules as well as the atoms, e.g.
Cl2.
Deflection
Mass spectra can be used to find the
Mr of molecules. The peak with the
greatest m/z corresponds to the Mr, but
beware of isotopes.
70 (35Cl-35Cl)+
43
4)
72 (37Cl-35Cl)+
Detection
35 (35Cl)+
29
58
74 (37Cl-37Cl)+
37 (37Cl)+
10
20
30
40
50
60
m/z
© www.CHEMSHEETS.co.uk
19-Feb-12
70
80
90
100
110
10
20
30
40
50
60
70
80
90
100
110
m/z
Chemsheets AS 002
1)
b) On the diagram, sketch the path of the following ions under the same
conditions:
The mass spectrum of
a compound is shown.
What is the relative
molecular mass of the
compound?
4)
i)
24
Mg ion labelled P
ii)
26
+
Mg ion labelled Q
iii)
25
Mg
+
2+
ion labelled R
a) Calculate the relative atomic mass of lead given the mass spectroscopy
data below.
b) Identify the species responsible for the peak at m/z 208.
2)
The mass spectrum of a compound is shown. What is the relative molecular mass of
the compound?
5)
6)
3)
25
+
a) The diagram shows the path of a Mg ion through a mass spectrometer. Name
the processes taking place at A, B, C and D.
7)
© www.CHEMSHEETS.co.uk
19-Feb-12
m/z
204
206
207
208
relative intensity
0.287
4.51
4.32
10.00
Calculate the relative atomic mass of chromium given the mass spectroscopy
data below.
m/z
50
52
53
54
relative abundance (%)
4.3
83.8
9.5
2.4
Calculate the relative atomic mass of krypton given the mass spectroscopy
data below.
m/z
78
80
82
83
84
86
relative abundance (%)
0.3
2.3
11.6
11.5
56.9
17.4
Calculate the relative atomic mass of strontium given the mass spectroscopy
data below.
m/z
84
86
87
88
relative intensity
0.00678
0.120
0.085
1.000
Chemsheets AS 002